In Schizosaccharomyces pombe the onset of mitosis is regulated by a network of protein kinases and phosphatases. The M-phase inducing Cdc2-Cdc13 cyclin-dependent kinase is inhibited by Wee1 tyrosine kinase and activated by Cdc25 phosphatase. Wee1 is negatively regulated by Nim1 protein kinase. Here, we describe investigations aimed at better understanding the role of Nim1 in the mitotic control. The most important finding to emerge from these studies is that Wee1 and Nim1 have different patterns of intracellular localization. Immunofluorescence confocal microscopy has revealed that Nim1 is localized in the cytoplasm, whereas it substrate Wee1 is predominantly localized in the nucleus. Previous studies showed that the Cdc2-Cdc13 complex is located in the nucleus. Diversion of Nim1 to the nucleus, accomplished by addition of the SV40 nuclear localization signal, caused the advancement of M, confirming that Nim1 has restricted access to Wee1 in vivo. We propose that the intracellular distribution of Nim1 and Wee1 may serve to coordinate the regulation of nuclear Cdc2-Cdc13 with cytoplasmic growth
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